Richie Merritt was a Baltimore high school student with no acting experience when he was improbably cast in "White Boy Rick."

So the movie (in theaters Sept. 14) required a steep learning curve for the 15-year-old – that started with figuring out just who his famous co-star, Matthew McConaughey, was.

Merritt admits that the Oscar-winning actor wasn't on his radar.

"I didn’t know who he was at first. Because I'm used to watching action movies with, like, Will Smith or Mark Wahlberg, stuff like that," says Merritt, now 17 and a high school senior. "But when I met him, it was like, 'I do know that guy.' I didn’t know his name was Matthew McConaughey."

No offense was taken by McConaughey or director Yann Demange. That Merritt didn't know the Hollywood star made his portrayal of inner-city teenager Richard Wershe Jr., the FBI's youngest-ever informant in 1980s Detroit, all the more authentic.

The real-life Wershe was paroled in 2017 after serving nearly three decades behind bars and is now serving time in Florida for a crime he was convicted of while in prison.

Richie Merritt, left, did not know his co-star before "White Boy Rick." The actor's name was Matthew McConaughey (R).(Photo: Scott Garfield, )

With McConaughey, 48, as his flawed but loving working-class father, Richard Sr., the filmmakers wanted an unknown to play his son. Talent scouts searched boxing gyms, community centers and high schools in five cities before finding Merritt at the principal's office.

After two weeks of training with an acting coach, his first, Merritt flew to Los Angeles to audition with McConaughey and won the pivotal role.

"If we go with Matthew McConaughey and a Hollywood kid, it doesn’t have authenticity. Richie brings a special emotional truth," Demange says. "It was a big chance. The studio was nervous. But I was convinced he was the one. And the support of Matthew helped."

McConaughey made that clear with studio executives and during the cast's first script reading.

"He was like, 'I got you and you got me,' " Merritt recalls. "And I told him, 'If you swing, I swing. I got your back, you got my back.' He was just real cool. And he was always there for me."

Merritt acknowledges there were times when he was overwhelmed thinking about the role

"I'd remember what my mother always told me: 'God wouldn’t put you here to fail,' " he says. "I would take a couple of deep breaths when I was anxious. I would always try to be in the moment, have fun and do what God brought me to do."

Merritt has relaxed enough to make a full-time run at acting. "It's most definitely something I plan to take on." But first he celebrated his movie debut at the Toronto International Film Festival last week.

"The producers were telling me, 'You did the hard part already, and now it’s time to go have some fun,' " Merritt says. "So I’m trying to have to some fun."

Richie Merritt is done with the hard part of making his acting debut in "White Boy Rick."(Photo: SCOTT GARFIELD/SONY)